Nephelometry is a technique used for measuring the concentration of particles suspended in a liquid sample. A conventional nephelometer includes a light source, a sample container, and an optical detector at a predetermined position and orientation (usually 90 degrees) with respect to the light source and the sample being tested. The optical detector produces a relatively low-level analog signal in response to the presence of light-scattering particles suspended in the liquid media, with the back-scatter signal being proportional to the concentration of the particles.
In designing a nephelometer, care has to be taken that no direct light from the source and no stray light originating at surfaces can reach the optical detector. Such light would elevate the background signal, making it impossible to detect very low particle concentrations. U.S. Pat. No. 5,872,361 discloses a nephelometer design whereby a non-imaging concentrator is positioned in-between the light beam and the optical detector, allowing excellent collection of scattered photons, but blocking un-wanted contributions from the device surfaces. U.S. Patent Publication No. 2003/0214653 discloses a nephelometer arrangement of internal surfaces, optical surfaces, and light baffles to the field of view of both the illumination beam and the detector means that significantly improves the lower detection limit by reducing signal contributions due to stray light.
Nephelometry techniques are used in different fields of life sciences, such as, immunology and clinical microbiology, but also in environmental sciences for monitoring air pollution or the clarity of drinking water. The dominant application of nephelometry in clinical microbiology is related to monitoring a microorganism inoculum concentration during sample preparation for microbial growth studies, such as, antimicrobial susceptibility testing (“AST”). The standard method of estimating the concentration of a microorganism in a sample is based on obtaining a quantitative value known as a McFarland value. McFarland standards are known concentrations of microbial solutions that can be used to prepare a standard curve in order to determine the concentration of particles in a sample, such as, e.g., the concentration of bacteria in a positive blood culture sample. In addition, in the clinical lab, there is a need to rapidly and accurately identify and characterize microorganism(s) in a multitude of samples in order to provide appropriate guidance to the physician in a timely manner. These identification and characterization methods often require the determination of the concentration of the microorganism in the sample prior to such testing.
Most nephelometers are equipped with only one illumination and detection path, and can accept only one sample container at a time. For high-throughput applications, including a multitude of sample containers such as a multi-well micro-titer plate, it is very time consuming to measure the concentration of the suspended particles in each sample in serial mode fashion.
Efforts have been made to provide a nephelometer with increased through-put. U.S. Pat. No. 5,969,814 discloses a nephelometer for interrogating a multitude of sample containers that are mounted on a revolving platform or carousel. In order to measure the concentration of suspended particles in a particular sample container, the carousel rotates until that container is aligned with an illumination and detection assembly for sample interrogation.
A nephelometer containing a circularly arranged array of optical cuvettes, mounted on a fast moving rotor assembly, is disclosed in U.S. Pat. No. 4,509,856. In this design, the nephelometric cuvettes themselves pass a location where they are interrogated by two illumination beams. Light emerging from the cuvette at angles of 90° and 180° relative to the illumination beams is measured by an optical detector.
EP Patent No. 0950892 discloses a nephelometer for interrogating an array of liquid sample containers that are arranged in a standard 96-well micro-titer plate format. The disclosed apparatus is equipped with a measuring head equipped with a light source and an optical detector to measure the transmitted light in a single well. The measuring head and the micro-titer plate are then moved in relation to each other, so that the optical density of each sample is measured in serial mode.
U.S. Pat. No. 4,685,801 discloses a multi-channel nephelometer wherein light from a single source is coupled into twelve illumination fibers simultaneously to optically interrogate twelve sample containers. Light transmitted from the sample containers is directed into one of twelve collection fibers. The emission ends of these collection fibers are disposed in a circular arrangement in a support member. The support member is then rotated so that light emitted from these fibers is directed to a photo diode sensor in serial fashion.
The above-described nephelometers fail to provide a device that is capable of determining the concentration of suspended particles in sample without the need for serial mode detection and the complicated mechanical devices needed to effect such detection. Accordingly, there is a need for a nephelometer that is capable of simultaneously determining the concentration of suspended particles in numerous samples. In addition, the nephelometer must be capable of detecting very low particle concentrations. In order to do so, background signal must be minimized. Consequently, improvement in automated, high throughput nephelometers continues to be sought.